Complex oxides can be produced by several processes that differ in terms of production cost and product properties. The oldest and most common such process involves the calcination of a reactant mixture in a furnace at temperatures of up to 1400° C. for periods of 2-24 hours (Valenzuela, “Magnetic Ceramics,” Cambridge University Press, p.311, 1994; Segal, “Chemical Synthesis of Advanced Ceramic Materials,” Cambridge University Press, Cambridge, p.182, 1989). In some cases, due to incomplete conversion, the sintered material requires grinding and additional calcination. This process requires an expensive high-temperature furnace and high-energy consumption. Furthermore, the product particle size is often rather large due to the long processing time at high temperatures, thereby requiring extensive size reduction to enable manufacturing of desired devices.
In contrast to the above-described calcinations processes, several wet chemical methods produce a more homogeneous product and enable better control of the particle size, which enables production of final components with superior properties (Rao, “Chemical Approaches to the Design of Oxide Materials,” Pure & App. Chem., 66(9), 1765-1772, 1994). These methods include co-precipitation (U.S. Pat. No. 5,200,390), sol-gel (Brinker et al., “Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing,” Academic Press, p.908, 1989), spray-drying (Masters, Spray Drying Handbook, 4th ed., George Godwin Limited, London, p.696, 1985), aqueous combustion-synthesis (Aruna et al., “Combustion synthesis and properties of strontium substituted lanthanum manganites La1-xSrxMnO3 (0≦x≦0.3),” Journal of Materials Chemistry, 7(12), 2499-2503, 1997; Mukasyan et al., “Perovskite membranes by aqueous combustion synthesis: synthesis and properties,” Separation and Purification Technology, 25, 117-126, 2001), hydrothermal (Barrer, “Hydrothermal Chemistry of Zeolites,” Academic Press, London, p. 360, 1982; Hirano, “Hydrothermal synthesis and characterization of ZnGa2O4 spinel fine particles,” Journal of Materials Chemistry, 10(2), 469-472, 2000), citrate precursor (Singh et al., “Low-temperature Synthesis of Mn0.2Ni0.2Zn0.6Fe2O4 Ferrites by Citrate Precursor Method and Study of their Properties,” Physica Status Solidi, 201, 7, 1453-1457, 2004), forced hydrolysis method (Kandori et al., “Roles of Metal Ions in the Formation of Hematite Particles from Forced Hydrolysis Reaction,” Ind. Eng. Chem. Res., 39, 2635-2643, 2000), microemulsion (Vestal et al., “Magnetic spinel ferrite nanoparticles from Microemulsions,” Int. J. of Nanotechnology, 1(1/2), 240-263, 2004), and cryochemical (Tretyakov et al., “Recent progress in cryochemical synthesis of oxide materials,” J. Mater. Chem., 9, 19-24, 1999). The wet-chemical methods generally require, however, calcination at a high temperature to obtain a powder product with the desired composition and structure. Additionally, while the wet chemical processes can produce high quality powder, the production costs are usually much higher than those involving calcination.
Another method of producing oxides composites is self-propagating high-temperature synthesis (SHS), also referred to as combustion synthesis (Merzhanov et al., “Method for making a composite,” U.S. Pat. No. 4,988,480, issued Jan. 29, 1991; Merzhanov, “The Chemistry of Self-propagating High-temperature synthesis,” Journal of Mater. Chem., 14, 1779-1786, 2004; Borovinskaya, “Chemical classes of the SHS processes and materials,” Pure & App. Chem., 64(7), 919-940,1992; Munir et al., “Synthesis of High-Temperature Materials by Self-propagating Combustion Methods,” Am. Ceramic Soc. Bull., 67(2), 342-349, 1988; Nersesyan et al., “Self-Propagating High-Temperature Synthesis of Ferrites,” Inorgan. Mater, 29(12), 1506-1508, 1993; Varma et al., “Combustion Synthesis of Advanced Materials: Principles and Applications,” Advances in Chemical Engineering, 24, 79, 1998). The product particle size is typically smaller than that produced by calcinations, but larger than that attained in several wet-chemical methods. In the SHS process, a highly exothermic reaction between a metal powder (such as Fe, Ti, Zr, Al, Mg, Ni, Cu, Hf, Nb, etc.) and an oxidizer generates a high temperature (e.g., on the order of ca. 2000° C.) front that propagates through the reactant mixture converting them to products. The solid-state reaction continues after the combustion in the post-combustion zone. SHS may also be conducted in a thermal explosion mode, in which the reactants are heated uniformly and the combustion is initiated simultaneously on the whole surface of the sample. The major advantages of SHS of oxides composites are: (i) low external energy consumption and no need for a high temperature furnace; (ii) a very short reaction time (the typical combustion velocity is from a few millimeters to a few centimeters per second); (iii) the possibility of synthesizing simple and complex oxide compounds; (iv) versatility of reactor design and simplicity of the entire process; (v) products that often have superior properties over those synthesized by other methods; and (vi) possible in situ densification to form desired parts or articles.
Reported synthesis of complex oxides by SHS include, superconducting oxides (Merzhanov et al., “Method of manufacturing oxide superconductors using self-propagating high-temperature synthesis,” U.S. Pat. No. 5,064,808, issued Nov. 12, 1991; Lin et al., “SHS of YBa2Cu3O6+x Using Large Copper Particles,” Physica C, 218, 130-136, 1993), magnetic ferrites (Avakyan et al., “Phase-Formation During the SHS of Barium Ferrites,” Int. J. of SHS, 1(4), 551-554, 1992, Avakyan et al., “New Materials for Electronic Engineering,” Amer. Ceram. Sos. Bull, 75(2), 50-54, 1996; Martirosyan et al., “Synthesis of Lead Ferrite in a Combustion Mode,” Int. J. of SHS, 10(2), 193-199, 2001, Martirosyan et al., “Phase-Formation during Self-Propagating High-Temperature Synthesis of Ferrites,” Inorgan. Mater., 38(4), 400-403, 2002), fuel cell components (Ming et al., “Reaction Steps and Microstructure Formation during SHS of La0.8Sr0.2CrO3,” Combust. Sci. and Tech., 128, 279-294, 1997, Ming et al., “Chemical Rate processes Involved in SHS of La0.9Sr0.1CrO3,” Int. J. of SHS, 7(4), 457-473, 1998; Ming et al., “Combustion Synthesis and characterization of Sr and Ga doped LaFeO3,” Solid State Ionics, 122, 113-121, 1999, Ming et al., “A new route to synthesize La1-xSrxMnO3,” J. Mater. Sci., 35, 3599-3606, 2000), catalysts (Xanthopoulou G., “Oxidative dehydrodimerization of methane using manganese based catalysts made by self-propagating high-temperature combustion synthesis”, Chem. Eng. Technol. 24 (10), 1025-1034, 2001), and glass ceramics and mixed-oxide materials (Yi et al., “Method of manufacturing aluminoborate glass-ceramic composite,” U.S. Pat. No. 5,792,417 issued Aug., 11, 1998, Yi et al., “Combustion synthesis of glass (Al2O3—CaO—X—Y) ceramic (TiB2) composites,” U.S. Pat. No. 6,645,424, Nov., 11, 2003; Pejryd et al., “Method for preparing ceramic mixed-oxide materials, particularly intended to be used as matrix material in composite ceramic products,” U.S. Pat. No. 5,607,887, issued March 4, 1997; Sekhar et al., “Manufacture of net shaped metal ceramic composite engineering components by self-propagating synthesis,” U.S. Pat. No. 5,188,678, issued Feb., 23, 1993).
When the pure metal powder (Fe, Ni, Ti, Zr, Ta, etc.) that is used as a fuel component in the green charge is considerably more expensive than their other precursors, such as carbonates, oxides, nitrates, etc., production by SHS may be more expensive than calcination processes. Moreover, SHS cannot be used when the pure metal combustion is not highly exothermic, or when the fine powder of pure metal is either highly pyrophoric (e.g., lithium and lanthanum) or melts at room temperature (e.g., gallium). In addition, melting of the metal powder during SHS may have a deleterious impact on the product homogeneity. In addition, if some reactants are hygroscopic it may be difficult to mix them adequately by ball milling, and preparation and storage of the metal powders may affect the reactivity of the combustion (Gol{grave over ( )}dshleger et al., “Laws Governing Ignition and Combustion of Zirconium: II. Activated Combustion of Zr in Nitrogen,” Fizika Gorenia i Vsryva, 13, 783, 1977; Gol{grave over ( )}dshleger et al., “Trends in the Ignition and Combustion of Zirconium: I. Effects of Previous Treatment on Ignition in Oxygen,” Fizika Gorenia i Vsryva, 13, 257, 1977). For example, freshly prepared Ti powder is pyrophoric and ignites at room temperature. It well known that a thin oxide layer forms on the surface of metal particles, affecting their oxidation reactivity. For example, Rode and Hlavacek, (Rode et al., “An Experimental Study of Titanium Powder Reactivity in Gaseous Environments. Part I: Oxidation,” Combust. Sci. and Tech. 99, 143-160, 1994; Rode et al., “An Experimental Study of Titanium Powder Reactivity in Gaseous Environments. Part II: Nitridation,” Combust. Sci. and Tech. 99, 161-177, 1994) reported that titanium powder aging in oxygen and nitrogen affected their combustion properties.
Thus, while SHS of oxides has many advantages over calcinations and wet chemical methods, it still has limitations, particularly with regard to production costs and material compatibility. Accordingly, an improvement on SHS that reduces production costs and makes it more universally applicable, would be a welcome advance.